Silicon processing technology has been rapidly developing with advances in integrated circuits, and is adapted for production of MEMS. An SOI substrate prepared by adhering an active Si layer over an Si support substrate using a silicon oxide film (bonding oxide film, BOX film) can reduce the thickness of the active Si film. Thus, dielectric isolation high performance Si element can be manufactured. Silicon oxide film can be selectively removed by diluted fluoric acid, etc., and thus SOI substrate can be used for manufacturing MEMS having movable part. An SOI substrate is generally made by thermally oxidizing at least one of a pair of Si substrates and bonding them via silicon oxide film by thermocompression.
To meet requirements of size reduction and performance enhancement for high-frequency (RF) parts of mobile phones etc., research and development on RF signal switches using MEMS technology are vigorously performed. An MEMS switch can be manufactured by patterning the active Si layer of an SOI substrate in a stripe shape and removing the bonding oxide film to form a flexible beam, and forming a movable contact on the flexible beam and a fixed contact above the flexible beam. The flexible beam may have either a cantilever (single-end supported) beam structure or a both-end supported beam structure. Since a MEMS switch is a mechanical switch, it can keep the parasitic capacitance small, is accompanied with smaller loss, has higher insulation property, and has better distortion characteristics for signals, compared to a semiconductor element-based switch.
It is possible to form a flexible cantilever beam by patterning the active silicon layer of an SOI substrate into a cantilever beam shape and removing the BOX film under the cantilever beam by etching. It is possible to constitute a switch by forming a movable electrode on the cantilever beam and a fixed electrode extending above the movable electrode and making the cantilever beam deformable upwards. As means for deforming the flexible cantilever beam upwards, there are known method using a piezoelectric actuator, method using an electrostatic actuator, etc. (for example, Japanese Laid-Open Patent Publication (JPA) No. 2006-261515).
As illustrated in FIG. 11A, it is possible to form an elastic cantilever beam CL by patterning an active silicon layer AL provided with an insulation layer on its surface and removing the bonding oxide film BOX thereunder. A movable contact electrode MCE is formed on the top end of the front surface of the cantilever beam CL, and at the same time an underlying electrically conductive layer is formed in a desired region. A fixed contact electrode FCE extending to the movable contact electrode MCE from a fixed part is formed to provide a pair of contacts for the switch. In the region from the proximal end of the cantilever beam CL to an intermediate position, a piezoelectric actuator PEA, including a lead zirconate titanate (PZT) or other piezoelectric material layer PEL sandwiched between a pair of driving electrodes, LE and UE, is formed. Plated metal layers PL1, PL2 and PL3 are formed in the terminal regions. A bias voltage source V is connected between plated metal layers PL2 and PL3.
When a voltage V is applied between the electrodes UE and LE of the piezoelectric actuator PEA from the bias voltage source V as illustrated in FIG. 11B, the piezoelectric material layer PEL increases its size in the direction of the electric field (thickness) and at the same time reduces its in-plane size to maintain the volume constant.
As illustrated in FIG. 11C, reduction in the in-plane size of the piezoelectric actuator PEA applies a contractive stress in the top surface of the cantilever beam CL. The cantilever beam CL is deformed to bend or warp upwards. Extending the proximal end of the piezoelectric actuator PEA from the fixed end of the cantilever beam CL towards the fixed region of the active silicon layer will enlarge the upward displacement of the cantilever beam CL at the other end.
As illustrated in FIG. 11B, as the cantilever beam CL warps upward, the movable contact electrode MCE comes in contact with the fixed contact electrode FCE to turn on the switch. When the application of a voltage to the piezoelectric material layer is stopped, the contractive stress disappears, and the cantilever beam CL loses warping due to the elasticity of the cantilever beam CL. The movable contact electrode MCE is detached or separated from the fixed contact electrode FCE, to turn off the switch.
In place of a piezoelectric drive mechanism, an electrostatic drive mechanism may be used. A movable electrode is formed on the top surface of the flexible beam and a fixed electrode is formed above the movable electrode to constitute a switch having an electrostatic drive. The flexible beam can be displaced upward by electrostatic attraction between the electrodes, to close the contacts, thereby turning on the switch.
If a MEMS switch repeats such on/off motions numerous times, such phenomenon as called sticking can occur that closed contact points can not be separated. The MEMS switch cannot be turned off by the elastic restorative force of the beam. Sticking may more easily occur as the elastic restorative force of the beam is small. To prevent sticking, it is desirable to increase the elastic restorative force of the beam. However, it is desirable to reduce the drive voltage (turn-on voltage) of the switch. For lowering the drive voltage of the switch, it is advantageous to decrease the elastic restorative force of the beam as low as possible.
For reducing the turn-on voltage and preventing sticking, it can be considered to introduce a drive mechanism for separating the closed contacts. For example, it is possible to provide a flexible beam with a piezoelectric drive mechanism and an electrostatic drive mechanism and to use one for closing the contacts and to separate the closed contacts (for example, JPA No. 2007-35640).
As illustrated in FIG. 12, a flexible beam CL is projected from support SP disposed on a support substrate SS, a movable contact electrode MCE and a movable driving electrode MDE are disposed on a lower surface of the flexible beam CL, and a fixed contact electrode FCE and a fixed driving electrode FDE are disposed on an upper surface of the substrate in face-to-face relation. The switch can be turned on by applying a voltage between driving electrodes MDE and FDE, to close the contact electrodes MCE and FCE by electrostatic attraction.
A piezoelectric material layer PEL is formed on an upper surface of the flexible beam CL and opposing (inter-digital) comb-shaped electrodes CEA and CEB are formed on an upper surface of the piezoelectric material layer, to form a piezoelectric drive mechanism for turning off the switch. When turning the switch off, not only the voltage applied between the driving electrodes MDE and FDE is turned off, a voltage is additionally applied between the comb-shaped electrodes CEA and CEB to generate a force which contracts the piezoelectric material layer PEL, and warp the flexible beam CL upward, to positively separate the contact electrodes MCE and FCE.
By providing two drive mechanisms as described above, it is possible to generate a force which separates the contact electrodes when the switch is to be turned off, thus preventing sticking. However, with the configuration illustrated in FIG. 12, it is necessary to form opposing electrodes in narrow gap between the flexible beam and the substrate. This puts severe restriction on the manufacturing processes. In the case of forming a flexible beam through the use of the active Si layer of an SOI substrate, the configuration will be difficult to realize.